The present invention relates to accelerators for fishing jars. The invention has particular application in accelerators which use a compressible fluid to accelerate the jarring action.
Conventional accelerators for fishing jars generally include a mandrel, that is telescopically arranged with an outer housing, and a fluid filled chamber, which is positioned between the mandrel and the housing. In these accelerators, the volume of the fluid chamber decreases as the mandrel telescopes out of the outer housing. This chamber is filled with a compressible fluid that enables the acceleration of the jarring action when the compressed fluid expands after the jar has tripped. In the simplest of these accelerators, the compression chamber, that is filled with a compressible fluid, is sealed by upper and lower seals. These seals are located between the mandrel and housing and prevent fluid from flowing out of or into the fluid chamber.
In such an accelerator, the upper and lower seals may be lubricated on the sides exposed to the fluid within the fluid chamber. Their other sides, however, may be exposed to the mandrel's and housing's abrasive nonlubricated surfaces that lie outside of the fluid chamber. For example, the section of the accelerator on the upward side of the upper seal may be exposed to drilling mud, rather than a fluid having better lubricating properties. After the jar has tripped, and the mandrel begins to slide downward within the accelerator housing, this upper seal rubs against the section of mandrel that is lubricated only with the drilling mud, causing the upper seal to come into contact with a relatively abrasive surface while still energized by the high pressure compressed fluid.
Similarly, the downward movement of the mandrel causes the lower seal to come into contact with a lower section of the mandrel which, like the upper section, is not as well lubricated as the surface of the mandrel that borders the compression chamber. This downward movement thus also causes the lower seal to contact a relatively abrasive surface. To prevent excess wear and to lengthen the useful lives of such working parts as these seals, it is desirable that they be lubricated on each side so that the seals that are subjected to high pressure differentials will remain properly lubricated whether the mandrel is moving upward or downward.
In these same conventional accelerators, the compressible fluid used to achieve the desired spring effect and at the same time maintain an economical tool length is usually a silicone oil. Silicone oil, in general, has a low bulk modulus compared to other hydraulic fluids or lubricating oils. By way of example, the bulk modulus of silicone oil is about 150,000 p.s.i., compared to about 265,000 p.s.i. for mineral based hydraulic fluids. However, the bulk modulus of silicone oil is significantly increased if the pressure of the oil is increased, as will occur when the tool is subjected to the hydrostatic pressure of an oil well. If the bulk modulus of the silicone oil is allowed to increase, the accelerator will become ineffective because it will lose much of its stroke.
Conventional accelerators attempt to overcome this weakness by isolating the silicone fluid from the hydrostatic pressure by confining the fluid in a closed chamber. This is only partially effective because the temperature of the well bore will, in general, increase in proportion to the hydrostatic pressure. If the temperature of the oil in a closed chamber is increased the pressure of the oil will increase in proportion if the oil is not allowed to expand. An advantage of the present invention is that it may provide an accelerator that effectively isolates the silicone oil from the hydrostatic pressure of the well bore and at the same time may provide an expansion chamber that prevents the increase in well bore temperature from increasing the pressure of the silicone oil, thereby providing an accelerator with an effective stroke under any expected combination of hydrostatic pressure and well bore temperature.
The effectiveness of the jarring action is related to the sum of the total stretch of the pipe above the jar plus the stroke of the accelerator. If the well is shallow or the fishing string is short there will be minimal pipe stretch, and under these conditions it is desirable that the accelerator begin to stretch open at a low pull. However, if the well is deep and the fishing string is long there will be significantly greater pipe stretch, and under these conditions it is desirable that the accelerator be able to resist a higher load before reaching the end of its stroke. It is a further advantage of the present invention that it may provide an accelerator that automatically varies its operating range in response to changes in hydrostatic pressure, thereby achieving an accelerator that is effective both shallow and deep without being excessively long.